1. Field of the Invention
The present invention relates to an assembling method of a magnetic sensor using magnetoresistive elements that produce an output signal in accordance with magnetism sensed.
2. Description of the Related Art
Conventionally, magnetic sensors using magnetoresistive elements each of which produce an output signal in accordance with magnetism sensed have been developed. Each magnetoresistive element used in such magnetic sensors basically has a lamination structure that is composed of a free layer (free magnetic layer), a non-magnetic layer, a fixed layer (pinned magnetic layer), and an exchange bias layer (antiferromagnetic layer). A bias magnetic filed of the exchange bias layer acts on the fixed layer, whereby the fixed layer is magnetized in a particular direction by the exchange bias layer. On the other hand, the magnetization direction of the free layer is varied by an external magnetic field.
To fix (pin) the magnetization direction of the fixed layer of each magnetoresistive element, it is necessary to adjust the lattice magnetization of the exchange bias layer. To this end, a heat treatment is performed in which the exchange bias layer is cooled in a state that a magnetic field in a prescribed direction is applied to the exchange bias layer and it is heated to a temperature that is higher than or equal to a temperature called a blocking temperature where an exchange anisotropy magnetic field disappears.
In magnetic sensors using such magnetoresistive elements, magnetoresistive elements in which the magnetization directions of the fixed layers form 180° (they are in opposite directions) are connected to each other in series and a bridge circuit is formed by those magnetoresistive elements. In this case, since the magnetization directions of the fixed layers of the magnetoresistive elements that are connected to each other in series should form 180° (they should be in opposite directions), it is necessary to magnetize the fixed layers in directions that form 180° (i.e., in opposite directions).
Conventionally, the fixed layers of magnetoresistive elements are magnetized in the following manner. Conductors for current conduction are provided for magnetoresistive elements that are formed on a single substrate. Currents are caused to flow through the conductors and magnetic fields generated by the currents are applied to the magnetoresistive elements, whereby their fixed layers are magnetized. In this method, it is difficult to magnetize the fixed layers by sufficiently strong magnetic fields because the amounts of currents that flow through the conductors are restricted. As a result, a strong exchange anisotropy magnetic field cannot be obtained and the absolute value of an output signal of each magnetoresistive element cannot be made large. There is another problem that when a magnetic sensor is used in a variable resistor, there occur large deviations from a standard variation characteristic (i.e., a magnetic field variation vs. output variation characteristic). The reason for this problem would be that with a weak exchange anisotropy magnetic field it is difficult to magnetize each fixed layer in a predetermined direction.
Another method to replace the above conductor magnetization method has been developed in which the fixed layers of magnetoresistive elements are magnetized by applying an external magnetic field to the magnetoresistive elements. With this method, magnetization can be effected by a strong external magnetic field. However, since magnetoresistive elements adjacent to each other should be magnetized in directions that form 180° (i.e., in opposite directions), an external field that is applied to one magnetoresistive element affects the other magnetoresistive element; a strong limitation is imposed on the strength of the magnetic field for magnetization. As a result, it is difficult to magnetize the fixed layers of magnetoresistive elements by a sufficiently strong magnetic field. Therefore, the absolute value of an output signal of each magnetoresistive element cannot be made large. For the same reason as described above, there occur large deviations from a standard variation characteristic when a magnetic sensor is used in a variable resistor.
In the above-described conventional example, the fixed layer is generally made of α-Fe2O3 and the strength of the magnetic field is about 200 kA/m. Nowadays, it is required to increase the exchange anisotropy magnetic field by using PtMn or the like for the fixed layer and magnetizing it at about 600 kA/m. However, where a bridge circuit is formed by the conventional external magnetic field magnetization method, as described above, an external field that is applied to one magnetoresistive element affects the other magnetoresistive element, the strength of the magnetic field for magnetization is restricted to about 200 kA/m; it is substantially impossible to increase it to the required level of 600 kA/m. Therefore, the current situation is such that the absolute value of an output signal of each magnetoresistive element cannot be made large or there occur large deviations from a standard variation characteristic when a magnetic sensor is used in a variable resistor.